The present application relates to a method of operating a fuel cell power generator, and a fuel cell power generator to be operated according to the method.
Fuel cells are power generating elements which are configured to carry out power generation by electrochemically reacting a fuel such as hydrogen or methanol with an oxidizing gas such as oxygen gas. They receive attention as power generating elements that do not invite environmental pollution, because power generation therein yields water as a product. Attempts have been made to use fuel cells, for examples, as electric power supplies for driving automobiles.
Fuel cells are classified as various types typically by electrolyte, and representative examples thereof are fuel cells using solid polymer electrolytes as electrolytes. Such solid polymer electrolyte fuel cells can be obtained at low cost, can easily come down in size, thickness, and/or weight, and show high output densities as their cell performance. Accordingly, they are promising as electric power supplies for driving electronic equipment. Some of solid polymer electrolyte fuel cells use hydrogen as a fuel. There have also been developed solid polymer electrolyte fuel cells using methanol or natural gas as a fuel, which fuel is reformed into hydrogen. In addition, direct methanol fuel cells (DMFCs) have been developed, in which methanol as a fuel is directly fed to fuel cells, and power generation is carried out.
Direct methanol fuel cells each have a power generation cell including an electrolyte, such as a solid polymer electrolyte, sandwiched between a cathode and an anode. In direct methanol fuel cells, an aqueous methanol solution is fed to the anode so as to cause a reaction between methanol and water on the surface of the anode, which reaction is represented by: CH3OH+H2O→CO2+6H++6e−. Air is fed to the cathode so as to cause a reaction on the surface of the cathode among oxygen in the air, and an electron (e−) and a proton (H+) formed in the anode, which reaction is represented by: 3/2O2+6H++6e−→3H2O. Specifically, methanol as a fuel, if used alone, does not act to proceed a reaction. Accordingly, both methanol and water should be fed as a mixture to the anode.
Feeding processes herein include, for example, a process of mixing methanol and water to yield an aqueous methanol solution having an appropriate concentration, and feeding the aqueous methanol solution to an anode. This process may simplify the system. However, when methanol and water are fed as a mixture having an appropriate concentration, it is difficult to vary the concentration of the aqueous methanol solution in accordance with conditions in operation of direct methanol fuel cells. This is because a methanol concentration to yield a maximum output is generally different from a methanol concentration to yield a maximum efficiency in direct methanol fuel cells. Certain direct methanol fuel cells each include a membrane electrode assembly (MEA) using a solid electrolyte typified by Nafion®. In fuel cells of this type, the membrane electrode assembly deteriorates at a higher speed with an increasing concentration of methanol. The methanol concentration herein is increased in order to increase energy density.
Another feeding process is a process of mixing methanol in a high concentration with water in a fuel cell power generator to thereby yield an aqueous methanol solution having an appropriate concentration, and feeding the aqueous methanol solution to an anode. The water herein is formed in a cathode as a result of power generation. According to this process, the concentration of the aqueous methanol solution can be adjusted suitably according to conditions of a fuel cell power generator. To adjust the concentration, however, the concentration of the aqueous methanol solution should be detected or determined. To detect the concentration, there are used concentration sensors and cells therefor, according typically to a static capacitor method, an electrochemical oxidation method, an ultrasonic method, or a densimetric method.
These concentration sensors exhibit excellent properties when a fuel mixture includes methanol and water alone. However, direct methanol fuel cells may invite by-products such as formic acid as a result of side reactions of power generation reactions, and such by-products cause errors in measured concentrations. Accordingly, measured concentrations including some errors makes it difficult to feed an aqueous methanol solution having an optimum concentration. Fuel cells, if including sensing cells, generally show a varying output with time. The fuel cells thereby show different outputs with elapse of time during operation. In addition, histories of fuel cells affect the degree of variation in output of such fuel cells. Accordingly, it is difficult to continue detecting absolute concentrations.
There is reported a process of determining a concentration of an aqueous methanol solution without such sensors, feeding the aqueous methanol solution at a flow rate suitable to the concentration, and thereby optimizing the operation of a fuel cell power generator (for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-22830). According to this process, a concentration of an aqueous methanol solution is determined by calculation based on the output current and operating duration of the fuel cell power generator; the aqueous methanol solution is fed to a power generation unit at a flow rate varied depending on the determined concentration; and the fuel cell power generator is thereby operated in an optimum manner.
It is desirable to optimize the concentration of a fuel depending on an operation condition, so as to maximize output characteristics and electrical efficiency. It is difficult, however, to optimize the concentration of a fuel according to this process.
Japanese Unexamined Patent Application Publication (JP-A) No. 2003-22830 discloses yet another process. According to this process, the concentration of an aqueous methanol solution is determined by carrying out predetermined computations using an output current and an operating duration as parameters. The process does not use a sensor for detecting the concentration. Thus, the aqueous methanol solution is fed to a fuel cell at an optimum flow rate which is set based on the determined concentration. Specifically, the fuel cell is operated in an optimum manner set depending not on the concentration but on the flow rate of the aqueous methanol solution according to this process. The flow rate should therefore be controlled minutely. In addition, the determined concentration may have an error, because fuel cells tend to exhibit varying outputs with time, and histories of fuel cells affect the degree of the variation. Furthermore, complicated computations should be carried out so as to determine concentrations.
Additional features and advantages of the present application are described in, and will be apparent from, the following Detailed Description and the figures.